1. Field of the Invention
The present invention relates to a photomask inspection apparatus for inspecting photomasks (or also called reticles but herein simply called masks) used in a semiconductor manufacturing process.
2. Description of Related Art
Generally, there are widely known two mask defect inspecting method: an inspection method which compares a mask pattern and design data (generally called a die-to-database comparing method) and an inspection method which compares two mask patterns (generally called a die-to-die comparing method). In either of these inspection methods, an image of a mask pattern is detected with a microscope. At this time, if an optical microscope is used, the mask pattern needs to be illuminated with light. For the light source (i.e., a mask inspection light source), there are two main categories: the use of a lamp and the use of a laser. In photomask inspection apparatuses using a laser, a continuous laser that generates continuous laser light is usually used.
At present, photomask inspection apparatuses using continuous laser light of 257 nm in wavelength (the second harmonic of the wavelength of 514 nm that is the maximum output line of an argon laser) as the inspection light source laser are available in the market, which are described in, e.g., Proceedings of SPIE, vol. 5446, pp. 265-278, 2004 or Toshiba Review, vol. 58, No. 7, pp. 58-61, 2003.
As feature sizes of semiconductor devices become finer, patterns on the masks become finer. Accordingly, for improved sensitivity in detecting defects, there is a demand for use of a shorter wavelength for the light sources of photomask inspection apparatuses as well. The mask inspection light source of the next generation is required to be a light source having a wavelength of 200 nm or less. For example, a photomask inspection apparatus has been developed which has generated therein ultraviolet laser light having a wavelength of 198.5 nm that is the summation frequency of the frequencies of the second harmonic of an argon laser of 488 nm in wavelength and a fiber laser of 1064 nm in wavelength to use as the mask inspection light. Such a photomask inspection apparatus is disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 2006-73970 or Proceedings of SPIE vol. 5992, pp. 43-1-43-8, 2005.
In the structure of typical KrF or ArF lithography masks used in the semiconductor manufacturing process, one face of a mask substrate 801 made of synthetic quartz is a pattern surface 802 like a mask 810 shown in FIG. 13. Spacer 803a, 803b are provided on the periphery of the mask substrate 801. A pellicle 804, a transparent thin film, is applied onto the spacer 803a, 803b so that the pattern surface 802 is kept in a sealed space, thereby preventing dust outside the mask 810 from sticking to the pattern surface 802. Because the pellicle 804 is formed of an extremely thin polymer of about 1 μm in thickness, there is the problem that the pellicle is easy to be torn.
FIG. 14 shows the usual configuration of a conventional photomask inspection apparatus. As shown in FIG. 14, the conventional photomask inspection apparatus 800 observes an observed area in the pattern surface 802 with an object lens 807 placed directly above the pellicle 804 of the mask 810. Laser light L81 as illuminating light is reflected downward by a polarization beam splitter 805, passes through a quarter wavelength plate 806 to be converted to circular polarization, passes through the object lens 807, and is irradiated onto the observed area in the pattern surface 802. The illuminated observed area in the pattern surface 802 is enlarged by the object lens 807 and a projection lens 808 and projected onto a two-dimensional sensor 809. The way that illuminating light is made incident from the object lens 807 side is called reflected illumination. Meanwhile, the way that the pattern surface 802 is illuminated from the opposite side thereof from the object lens 807 is called transmitted illumination.
In the lithography technology of the semiconductor manufacturing technology, an ArF excimer laser of 193.4 nm in wavelength is widely used as the light source for exposure. An exposure technique using this is called ArF lithography. As lithography technology for realizing even finer feature sizes, an exposure technique called liquid immersion where the gap between the projection lens of an exposure apparatus and a wafer is filled with water is becoming widely used. This is also called ArF liquid immersion exposure, ArF liquid immersion, or the like. FIG. 15 shows the usual configuration of a conventional liquid immersion exposure apparatus. As shown in FIG. 15, in a liquid immersion exposure apparatus 900, the gap between a lens (not shown) at the lower end of a reduction projection optical system 903 used to project a reduced image of a mask 901 onto a wafer 902 and the wafer 902 is filled with pure water 904.
The wafer 902 is mounted on a wafer stage 905, and with the gap between the wafer 902 and the reduction projection optical system 903 being filled with pure water 904, the wafer 902 moves back and forth. The pure water 904 is supplied from a pure water supply unit 906 and sucked into a pure water sucking unit 907 so as to usually fill the space under the reduction projection optical system 903. The ArF liquid immersion exposure is described in, e.g., Electric Journal, pp. 73-74, May 2004, and an inspection apparatus for wafers or the like using a liquid immersion optical system is described in, e.g., Japanese Unexamined Patent Application Publication No. 2005-83800, No. 2005-338027, and No. 2006-171186.
In order to improve the resolving power, i.e., sensitivity of a photomask inspection apparatus, it is inevitable to use a light source of a shorter wavelength. In the future, even a laser of the above-mentioned wavelength of 198.5 nm will not suffice in sensitivity. Even if an ArF excimer laser of 193.4 nm in wavelength or the like can be used for the mask inspection light source, they will not be enough in sensitivity to inspect a 32-nm generation of masks.
Accordingly, considering the application of the liquid immersion technique to photomask inspection apparatuses as with the exposure technology, the problem below exists, and hence it has been extremely difficult to apply the liquid immersion technique. The reason is that, as seen from the conventional photomask inspection apparatus 800 of FIG. 14, if an attempt is made to apply the liquid immersion technique to photomask inspection, the space between the pattern surface 802 and the object lens 807 will be filled with water, and hence the pellicle 804 cannot be used. Of course, at a stage before applying the pellicle to them in the production process of masks, such liquid immersion technique can be used, but in the photomask inspection where masks finished by applying the pellicle thereto are inspected, the liquid immersion technique cannot be used. There is the problem that in the conventional photomask inspection apparatus, it is difficult to improve the resolution and sensitivity.
An object of the present invention is to provide a photomask inspection apparatus of high resolution.